I use the white silica sand. It is often sold as "play sand" for kids' sandboxes. it's coarse and the grains have sharp edges. I've been told it's actually crushed granite..?

I have a plastic garbage can with a lid that I keep my sand in and the can is in my storage shed so it doesn't see any rain. It stays really dry in there. If the sand somehow gets damp while I'm using it, I just throw it back in there and leave the lid off for a few days and it dries out.

Blasting sheet metal is always a risky proposition. Any media that generates heat can distort sheet metal.